1. Field of the Invention
The present invention relates to linear beam electron devices, and more particularly, to a low-power depressed-collector klystron that provides high efficiency and wide bandwidth.
2. Description of Related Art
Linear beam electron devices are used in sophisticated communication and radar systems which require amplification of a radio frequency (RF) or microwave electromagnetic signal. A conventional klystron is an example of a linear beam electron device used as a microwave amplifier. In a klystron, an electron beam originating from an electron gun is caused to propagate through a drift tube that passes across a number of gaps, each gap being part of a resonant cavity of the klystron. The electron beam is velocity modulated by a RF input signal introduced into the first one of the resonant cavities. The velocity modulation of the electron beam results in electron bunching due to electrons, that have had their velocity increased, gradually overtaking the electrons that have had their velocity decreased. The traveling electron bunches represent a RF current in the electron beam, which induces electromagnetic energy into subsequent resonant cavities. The electromagnetic energy may then be extracted from the last of the subsequent resonant cavities as an amplified RF output signal.
The bandwidth and efficiency of a klystron are both of considerable importance in klystrons. For example, the information rate of the signal the klystron can amplify increases with the bandwidth. Also, the power consumed by the klystron decreases as the efficiency increases.
The bandwidth of a klystron increases as the ratio of beam current to beam voltage increases, or rather, as the beam conductance is increased. This occurs because both the load conductance across the output cavity and the loading conductances that the beam produces on the intermediate cavities are proportional to the beam conductance. Therefore the quality factor (Q) for these cavities, which is a measure of the energy stored to the energy lost per cycle, decreases as the beam conductance is increased. Accordingly, bandwidth is also inversely proportional to Q.
The beam conductance is determined by the perveance of the electron gun, which produces it, and by the voltage at which the electron gun is operated. The perveance (K) is defined by the relationship between the beam current (I) and the beam voltage (V) as I=K V.sup.3/2. The perveance is generally 1.times.10.sup.-6 to 3.times.10.sup.-6 amperes per volt.sup.3/2 for the average klystron. The beam conductance (I/V) can thus be given by the expression I/V=K V.sup.1/2.
In low-power klystrons, the beam voltage is usually low and the corresponding power output is typically less than 1 kilowatt. One approach to increasing the bandwidth would be to increase the perveance because, as discussed above, increasing the perveance increases the beam conductance and thus the bandwidth. However, this approach has two disadvantages. First, if the perveance is made high, there is an adverse impact on the efficiency of the tube because the space charge forces in the beam increase and make it difficult to tightly bunch the electrons of the beam. Second, as the perveance is increased at constant electron beam power, the beam voltage must be decreased. This results in a decrease in the electron beam velocity because the electron beam velocity is proportional to the square root of beam voltage. Furthermore, the dimensions of the cavity gaps along the beam must be held constant in terms of electron transit time to maintain good coupling of the cavity gap fields to the electrons. Therefore, the dimensions of these cavity gaps may become extremely small in low voltage klystrons, which are designed to operate at very high frequencies, and this results in difficulties in constructing a suitable klystron.
Accordingly, it would be desirable to provide an efficient klystron for low-power wide-bandwidth applications that could be easily fabricated. Furthermore, it would be desirable to provide a design methodology that would allow construction of various low-power klystrons for specific applications having relatively low output power and high efficiency, but with a much wider bandwidth and utilizing larger, more easily fabricated parts than would be found in a klystron of standard design.